Ceramic powder useful in the manufacture of green and densified fired ceramic articles

ABSTRACT

A method of coating ceramic powders, e.g. Si 3  N 4 , with a sintering aid and of utilizing such coated powders to form green and sintered ceramic bodies is described. The method comprises treating a dispersion of the powder with a solution of a sintering aid forming compound at a pH such that the zeta potential of the ceramic powder is of opposite charge with respect to the sintering aid in solution such that the sintering aid forming material is chemisorbed as a uniform monomolecular layer on the surface of the ceramic powder.

This is a division of application Ser. No. 07/601,222 filed Oct. 19,1990 issued as U.S. Pat. No. 5,102,592.

FIELD OF THE INVENTION

This invention is directed to the preparation of treated ceramic powdersspecifically useful in the manufacture of green and fired dense articlestherefrom powders and more particularly to such preparation whichincludes the use of sintering aids formed on the ceramic powders.

BACKGROUND OF THE INVENTION

Extensive research has been done over the years in an effort toeconomically and reproducibly produce dense, sintered ceramic articleswith a uniform microstructure starting with ceramic powders. Generally,in order to achieve high density, ceramic powders of selected particlesize have been treated in various ways including, for example, pressuresintering at high temperature; the mixing of the powders with bindersfollowed by low temperature heat treatment to form a `green` ceramicarticle which is then sintered at high temperatures; and the mixing of aflux (sintering aid) with the ceramic particles to reduce the sinteringtemperatures. To reproducibly manufacture reliable crystalline ceramicarticles one must control the characteristics of the starting powder andthe forming process parameters such that compacts of uniform high greendensity with interparticle pore size typically no larger than a singleparticle are achieved. For reproducible densification with control overthe properties of the sintered ceramic one should also control thesintering variables such that pore removal occurs during sintering andgrain boundaries develop between the particles.

During powder processing in liquid vehicles, one generally requires astable particle dispersion, i.e. essentially free of agglomerates. Ithas been reported that the addition of surfactants to the liquid may beused to attain stability. Ellen S. Tormey, in "The Use of Surfactants inthe Processing of High-Technology Electronic Ceramics" states"Achievement of a stable dispersion requires the formation of repulsiveinterparticle forces. In aqueous systems, electrostatic repulsion isgenerally dominant and arises due to the interactions between theelectric double layers surrounding the dispersed particles. In nonpolarorganic media (e.g., hydrocarbons) stability arises due to repulsionbetween interacting molecules adsorbed onto the particle surfaces and isgenerally referred to as steric stabilization. As a general rule, in thelatter system, the most effective dispersants have strongly adsorbedfunctional groups and strongly solvated chains which extend into thesolvent. Systems which are stabilized by a combination of mechanisms(i.e., charge and steric) tend to be the most stable. Stericstabilization can be effective in both aqueous and nonaqueous media,whereas the electrostatic mechanism is generally only effective in wateror polar organic solvents. Most importantly, steric stabilization iseffective in dispersions containing high volume fractions of solids,typically used to process ceramics, whereas electrostatic stabilizationis generally only effective in dilute systems." She then teaches the useof such organic dispersants for use in particle size classification ofceramic powders. She further teaches "the dispersants typically used inceramics processing bond to the particle surfaces via hydrogen bondingor weak chemical bonding". She recognizes that "dispersants which canchemically react (couple) with the particle surfaces to form strongerbonds offer distinct advantages in powder processing. Formation of astrong surface chemical bond would ensure that the dispersant remains onthe particle surfaces during subsequent processing steps, resulting in asystem which is less sensitive to slight compositional or processingcondition variations. Coupling agent type dispersants would beespecially advantageous in tape casting systems, since they aremulticomponent and competitive adsorption is likely to be operative".Parish and Lalanandham have investigated the use of low molecular weightorganotitanates as dispersants for ceramic powders (BaTiO₃, Al₂ O₃) innonpolar organic solvents such as hexane and toluene. When usingcoupling agents as dispersants for electronic ceramics, the metallicportion of the molecule must not be detrimental to the electricalproperties of the ceramic or interfere with the sintering process, sincethis portion is not removed from the body during sintering. In the caseof a metal-oxygen linked organic chain, the dispersant will decomposeduring sintering to a metal oxide residue.

Forney further teaches that in theory, coupling agents can serve notonly as dispersants in ceramics powder processing, but also as dopantsand/or binders. Dopants (generally secondary metallic oxides) are oftenadded to ceramic powders to aid in the sintering process. For example,M_(g) O is a well known sintering aid for Al₂ O₃ ; likewise, Y₂ O₃ iscommonly used to enhance the densification of AlN. The addition of sucha dopant in the form of a coupling agent would ensure that it ishomogeneously distributed in the green body and also the sinteredceramic, since it bonds to the particle surfaces. For use as binders,coupling agents with polymerizable ligands can be synthesized. Such amolecule could first act as a dispersant and then be converted to abinder via an in situ polymerization step.

While much has been learned about the formation of high density sinteredceramic articles, the ability to economically and reproducibly producesuch articles utilizing some ceramics, e.g. silicon nitride (Si₃ N₄),has evaded scientists. Silicon nitride has numerous desirable physicaland chemical properties which, if it could be economically andreproducibly manufactured, especially in high density form, would makeit particularly attractive in both wear resistant and high temperatureapplications. High density Si₃ N₄ ceramic articles have heretofore beendifficult to form due to the largely covalent bonding and limited selfdiffusion, requiring not only high temperatures but pressure as high as1.5 GPa. Dense Si₃ N₄ has been obtained by the use of powdered additiveswhich, during the sintering process, provide a liquid phase to promotedensification. However, the previously employed additives also introduceunwanted secondary phases which deteriorate the high-temperaturemechanical properties of the densified material. Such prior art powderedadditives have generally been employed in amounts ranging from 5-20+weight % depending on the densification procedure, e.g. pressurelesssintering, high pressure sintering, hot-pressing, and hot-isostaticpressing. The additives have most often been added by ball or attritionmilling with the ceramic powder. This can adversely affect thecharacteristics and purity of the powder and distribution of theadditives is not uniform. Another method reported in the literature forintroducing the additive is the precipitation of the additive from adispersion containing the Si₃ N₄ powder, however, while the additive isintroduced somewhat more uniformly by this method, excessive shrinkageduring sintering was encountered.

The method of the present invention incorporates sintering aid additivesin a controlled, uniform, reproducible manner, providing a liquid phaseat the sintering temperature which is in uniform and intimate contactwith the ceramic powder grains, thus improving the kinetics fordensification and transformation and allowing for the economical andreproducible formation of densified green or sintered ceramics,including silicon nitride, from powdered starting materials.

SUMMARY OF THE INVENTION

The invention disclosed and claimed herein involves (1) the productionof treated ceramic powder particles characterized by an essentiallymonomolecular layer of a sintering aid or sintering aid forming compounduniformly chemisorbed on the surface of the powder particles; (2) theformation of a green ceramic article from the treated powder; and (3)the formation of a dense sintered ceramic article from either the greenceramic article or directly from the treated powder.

The treated ceramic powder is formed by (1) preparing a stabledispersion of a ceramic powder in a liquid media which contains, totallydissolved therein, a soluble compound which contains a metal ion whichcan form part of a sintering aid for said ceramic powder wherein saidceramic powder has a zeta potential which is displaced from theiso-electric point when at the pH of the liquid media and where thesoluble compound does not precipitate out of the solution at said pH andwhere the metal ion is chemisorbed onto the surface of the powder in amanner such that upon subsequent heating of the powder after separationfrom the liquid media, a uniform essentially monomolecular layer of thesintering aid is formed on the surface of the powder and (2) separatingthe treated powder from the liquid media leaving the chemisorbed layeron the powder.

The treated powder may be stored or sold in the form with thechemisorbed layer or may first be dried and/or heat treated to obtainthe final sintering aid thereon. One may continue processing the powderhaving the chemisorbed layer or sintering aid thereon such as by formingthe powder into a desired shape and heating to obtain a green ceramicarticle having a sintering aid uniformly absorbed on the powder particlesurfaces. Alternatively, one can fire the formed particles at theappropriate sintering temperatures and pressures before or afterformation of a green ceramic article to obtain a high density sinteredceramic product.

It should be understood that the treated powder as well as the greenceramic article or the high density sintered ceramic product is capableof being sold as an item of manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of Zeta potential in millivolts vs. solution pH forthree different silicon nitride powder samples.

FIG. 2(a)-(c) is a representation of one possible theoretical model fora powder-surfactant interaction leading to the formation of a uniformmonolayer of a metal-oxide sintering aid on the surface of the powderparticles employing an organometallic surfactant.

DETAILED DESCRIPTION OF THE INVENTION

Generally, in order to make a high density ceramic article from aceramic powder in accordance with the present invention one preferablyfirst disperses the powdered ceramic starting material in a liquid mediaand, while preventing agglomeration of the particles e.g. by ultrasonicvibration, adds a solution having a compound dissolved therein which ischemisorbable on the surface of the ceramic powder particles and which,when the particles are separated from the liquid media and heated, forman essentially monomolecular uniform layer of a sintering aid on thesurface of the powder. A preferred class of compounds for this purposeare organometallic surfactants. However, soluble inorganic salts such asthe nitrates of the desired metal-ion sintering aid is also suitable.The most suitable organometallic surfactants are preferably compoundswherein the metal ion does not form a fully coordinated complex with theorganic ligand of the surfactant and where the metal is stronglyassociated with an atom, such as an oxygen or nitrogen atom, on theligand. The oxygen (or nitrogen) atom strongly bonds to the surface ofthe ceramic, while the metal-ligand bond is a weaker bond that readilydissociates upon heating so that a uniform monolayer of a metal-oxide(or metal-nitride) sintering aid remains uniformly dispersed over thesurface of the powder particles. The organic ligand portion of thesurfactant is one which is associated with the solvent so as to besoluble therein. Further, the organometallic surfactant preferably formsa polymeric type of molecule in the solvent which promotes a uniformcoating of the metal-oxygen over the entire surface of each particle.

Whatever the chemisorbable compound may be, it is important that it havea charge in solution which is opposite the zeta potential of the ceramicpowder surface at the pH of the dispersion containing the powder and thedissolved chemisorbable compound, and that the pH be such that thechemisorbable compound is stable and does not precipitate from thesolution. By way of example, referring to FIG. 1 there is shown the zetapotentials of three different silicon nitride particle dispersions as afunction of pH. The particles are named according to their respectivesources. The Starck particles in the negative zeta potential range arethe most difficult to work with in that the pH to maintain negative zetapotential stability must be maintained at greater than about 8 and hencethe chemisorbable compound should show a positive charge and be stablein solution at pH's in excess of 8. This is considerably more difficultto achieve than if one utilized the Kemanord silicon nitride whereinnegative zeta potentials are attained at pH's in excess of 4 and hencethe chemisorbable compound need only be stable at pH's in excess of 4 tobe suitable. It should be noted that one could employ particles at pH'swherein the zeta potential is positive. However, when the zeta potentialis positive the sintering aid source should exhibit a negative charge insolution so as to readily chemisorb to the surface of the particle.

After exposure of the dispersed ceramic powder to the surfactant (orother chemisorbable compound), the powder is then separated from theliquid, such as by centrifugation followed by decantation of thesupernatant liquid, or by filtration. The powder may then be washed toremove excess surfactant so as to leave a uniform, thin (monolayer) ofthe surfactant over the ceramic particles. The coated particles are thenheated to a temperature which causes dissociation of the surfactant suchthat the ligand portion is removed leaving a substantially monomolecularlayer of sintering aid uniformly over the particles.

The particles may be formed into desired shapes from which a greenceramic is formed e.g. by slip casting, molding, pressing or any of themethods which are well known in the art either prior to or subsequent tothe dissociation of the surfactant. However, this is preferably doneprior to such dissociation. The green ceramic having each particleuniformly coated with sintering aid is then fired at high temperature,with or without pressure, to form a high density sintered ceramicarticle. Alternatively, one can fire the pressed treated particlesdirectly to form a high density article without first forming a greenceramic article.

A possible model which is proposed for the powder-surfactant interactionis shown in FIG. 2 for an organo-surfactant in an aqueous media havingan oxygen-metal bond which attaches to the surface of the ceramicpowder. R--C-- in FIG. 2 represents the organic ligand. As can be seen,the surfactant molecule shown in FIG. 2(a), forms a polymeric unit inthe aqueous media in which it is dissolved, as shown in FIG. 2(b). Asshown in FIG. 2(c), the oxygen atoms attached to the metal atom, M,which are remote from the body of the ligand, become strongly bonded tothe surface of the powder. Upon heating, the surfactant dissociatesleaving a uniform M-O-powder configuration. It should be understood thatthe model set forth herein is presented merely as a suggestedexplanation of the operation of the invention for an organometallicsurfactant chemisorbable compound and is not meant to limit theinvention to such model.

The following sets forth the preferred embodiment of the invention foruse in forming densified, high strength, silicon nitride (Si₃ N₄)ceramic articles. This is preferably accomplished by the use of a watersoluble cationic or non-ionic surfactant which contains a metal-oxygenbond which forms a metal-oxide sintering aid upon dissociation of thesurfactant molecule. Alternatively, one can employ a nitrate such aschromium nitrate as the source of the sintering aid for silicon nitride.The nitrate used should not precipitate out of the solution at theoperating pH. Hence, yttrium and magnesium nitrates are generallyunsuitable for use. The preferred surfactants are metallo-organiccomplexes having hydrated metal ions as an anchor group available forbonding to the surface of the ceramic powder. As previously indicated,the metal ions in the useful surfactants are preferably not fullycoordinated with the ligand since such fully coordinated metal-organiccoordination complexes give rise to weak hydrogen bonding with thepowder surface as opposed to the stronger chemical oxygen bondingobtained by use of the surfactants taught herein. These organicsurfactants provide greater suspension stability as compared with thenitrate as well as forming stronger bonds with the particle surface.Examples of suitable surfactants include, but are not limited to,methacrylate metal halide hydroxide, metal formates, metal lactates andmixtures thereof wherein the metal ion is preferably chromium, aluminum,magnesium or a mixture thereof. It should be understood that theparticular metal ion employed for forming the sintering aid depends uponthe particular ceramic powder composition. In the case of Si₃ N₄, thepreferred surfactant comprises, but is not limited to, a solution ofmethacrylate chromium chloride hydroxide, chromium acetate or aluminumlactate or mixtures thereof. This system is stable under normalprocessing conditions and strongly interacts with the Si₃ N₄ powdersurface, especially when such surface is predominantly a silonol surfacedue to exposure of the Si₃ N₄ to water or water vapor. Processing wasgenerally performed with a minimum amount of additives. The treatedpowders are then preferably first slip cast followed by sintering undervarious conditions. Sintering characteristics were assessed by density,E-modules, hardness, K_(c), x-ray diffraction, modulus of rupture andSEM.

EXPERIMENTAL General Procedure

Unless otherwise set forth the experimental results which followemployed the Kemanord powders referred to in FIG. 1 and sometimeshereinafter designated as powder A. Powder B is the UBE source powderwhose zeta potential is also shown in FIG. 1.

Si₃ N₄ powders having a BET surface area of about 10 m² /g weredispersed in deionized distilled water and exposed to ultrasonicagitation to break-up weak agglomerates. An aqueous surfactant solutionwas prepared and was then added to the ceramic dispersion. Thesurfactants employed were selected from methacrylate chromic chloridehydroxide, a chromium acetate or formate and an aluminum lactate andmixtures thereof. These surfactants are hereinafter designated as S1, S2and S3, respectively. The S1 monomer contains two chromium atoms,sixfold coordinated by an alkyl group, R, OH-groups, chloride and ashort chain-length radical. The complex is completely soluble in anaqueous medium. Dilution with water replaces the alcohol groups and partof the chlorine atoms with aquo groups. Hydrolysis causes more chlorineatoms to enter the aqueous medium as chloride ions. Once the aquo groupslose protons, hydroxyl bridges form. Increased growth by polymerizationmakes the molecules more positively charged, therefore resulting instrong chemisorbed bonds with powder particles having a negative zetapotential.

The described surfactant offers the advantage of both stabilizing thepowder dispersions and incorporating trivalent chromium oxide andaluminum oxide sintering aids onto the negatively charged particlesurface. FIG. 1(c) shows the predicted interface interaction; thepositive charge on the molecules makes the chromium bond to thenegatively charged surface. The chlorine atoms enter the dispersion aschloride ions. The presence of chlorine, especially related to reactionto HCl, in the final microstructure should be avoided. This can be doneby washing the treated powder. The S2 monomer contains three chromiumatoms and the S3 monomer is an aluminum surfactant having three aluminumatoms in the monomer.

The high melting points of chromium compounds makes it preferable to usean additional sintering aid. Magnesium or aluminum can be used to bothpromote formation of a liquid phase during sintering and give liquidimmiscibility, needed to control the final microstructure. Theseadditives also should be added as organic complexes to allow controlover both the colloidal chemistry of the system and the finalmicrostructure.

Addition of 0.15 mol % of the S1 Cr-complex to 1 gram silicon nitridepowder (S.A.=10M² /gr.), corresponding to an average calculated coverageof 1.5 to 2.0 sites/nm² resulted in a uniformly coated powder. Morecomplex resulted in an excess which remained in solution aftercentrifugation. Dispersions with 0.03-0.12 vol % can easily be preparedand remain stable for several weeks, once agglomerates have settled. Thestability of aqueous solutions of the S1 Cr-complex (as with other metalorganic surfactants) depends, among other things, on the pH and age ofthe solution. Solutions and, in this case, the dispersions, become moreacidic in time, because protons are produced during the polymerizationof the complex. Proper pH-control allows colloidal size molecules,without precipitation. If base is added in excess, or at the wrong timein the process (i.e. when the pH is too high), precipitation is morelikely to take place. By preparing the surfactant solution anddispersion separately and then mixing the two, better control over thepH of the Cr-containing solution is achieved and the possibility todeagglomerate the silicon nitride powders e.g. by ultrasonic agitation,as much as possible, is attained.

Upon addition of the dispersion to the complex solution, (or vice versa)the pH of the resulting dispersion changed to and remained at pH6.0-7.0. The pH of the initial dispersion, before addition to thecomplex solution, is important with respect to electrostaticstabilization and possible polymerization once the dispersion is addedto the complex solution. Polar surface sites or a slightly negative netsurface charge will promote interaction with the complex molecules. Thecationic group of the complex is expected to interact with the silanolsites at the particle surface. Both this and the possible precipitationat excess base impose certain requirements with respect to the powderdispersion. For example, the pH should be in excess of the iso-electricpoint of the silicon nitride powder and no base should be added to thedispersion since unwanted precipitation is likely to occur.

The pH of the dispersion was then adjusted with 10⁻² M NH₄ OH to a valueclose to the iso-electric point (IEP) value. The system was centrifugedand the supernatant, which contains any excess surfactant, was decanted.The powder was then redispersed in water and the pH adjusted away fromthe IEP value. Powders taken from slips before and after centrifugingwere used for mobility measurements in a 10⁻³ M KCl or KNO₃ aqueoussystem and for TEM, ESCA, FTIR, and thermal analysis (TA:TGA/DSC). Tocheck for the stability of the system centrifuging and redispersion ofthe powders was repeated up to five times and the pH was varied between3 and 11. Potentiometric titration was used to monitor the changes inthe surface potential of the powder while the surfactant solution wasintroduced. Slip viscosities were measured as a function of pH,temperature and surfactant. The slip was then cast into discs 25×10 mm.The discs were oxidized at 500° and 675° C. for up to 36 hours andanalyzed by ESCA and SEM. The discs were pressureless sintered at 1730°C. up to 6 hours under 0.1 MPa ultra-high purity nitrogen in a tungstencrucible and tungsten lined furnace, using a Si₃ N₄ powder bed. Hotisostatic pressing was also used at 1950° C. for 1 hour under 200 MPa ina BN powder bed. The sintered discs were analyzed by SEM, ESCA and XRD.The density (water immersion), microhardness (Vickers indenter),E-modulus (sonic velocity delay) and K_(c) (indentation) weredetermined. A few discs prepared from dry-pressing ball-milled mixturesof Si₃ N₄ powder, both with and without surfactant, and with high-puritymagnesia (SA=35 m² /gr) and alumina powder (SA=10 m² /gr) were alsosintered under 0.1 MPa nitrogen for 8 minutes at 1730° C. forcomparison.

Results

Most of the data was collected on powder designated, powder A while somedata was also collected on powder B. Transmission electron microscopy(TEM) showed significantly different powder morphologies. Powder A has alarge number of platelets, both sub-micron and up to 1-2 micron in size.Electron diffraction identified both the alpha and the beta phase. Inaddition, irregularly shaped particulates were observed. Powder B showedequiaxed particulates, but also contained a small amount of Si₃ N₄whiskers. Both powders behaved acidic in water, and stable dispersionsresulted at pH values in excess of 8. TEM samples prepared from theseelectrostatically stabilized dispersions showed mostly submicronagglomerates, but a number of considerably larger agglomerates waspresent as well. Samples taken of coated powders showed a smalleraverage agglomerate size, and hardly any large clusters were observed.

Addition to the concentrated surfactants (i.e. not in solution) loweredthe pH of the dispersions considerably due to initial polymerization ofthe surfactant molecules. Powder-surfactant interactions were obviousfrom potentiometric titration and acoustophoretic mobility data thatshowed a change in the sign of the powder mobility and the phase of themonitored signal upon addition of the surfactant, lowering the pH.However, surprisingly, increased interaction was obtained withsurfactants in solution. These solutions added to the powder dispersionshowed maximum interaction as indicated by rapid mobility and phasechanges. This was confirmed by ESCA data; the latter process gavepowders with consistent, reproducible amounts of additive. Precipitationof surfactant molecules due to the addition of excess base had to beavoided because precipitates would act as imperfections in the castmicrostructure. Precipitation occurred at a pH in excess of 8.4 for allof the surfactants. The IEP of the surfactants was around pH 7.5-8, andthe IEP of a coated powder dispersion shifted to these values. Exposureto excess acid or base during mobility measurements, as well as repeatedcentrifuging and redispersing, did not alter the IEP values nor the ESCAvalues. This behavior indicated that, once proper processing put thesurfactants onto the powder surface, they were stable against processingconditions. Potentiometric titration data indicated that formation ofhydroxyl bridges increased the net effective charge on a surfactantmolecule. That resulted in the higher degree of interaction with thepowder surface upon addition of base. This data also showed a saturationconcentration of surfactant added (0.15 mol/gr silicon nitride), inexcess of which the mobility did not further increase. Thisconcentration was used to minimize the amount of excess surfactant to beremoved by centrifuging and was used for the calculation of the amountof surfactant at the powder surface. The coated powders had highersurface potentials than as-received powders, and stable dispersions wereobtained containing up to 50 wt % solids.

Rheology measurements showed that slips of coated powders had lowerviscosity than electrostatically stabilized slips of as-received Si₃ N₄.The latter showed pseuo-plastic behavior for 65 and 68 wt % slips at pH9.3. Lowering the pH to 4 and 2.7 increased the viscosity of the systemorders of magnitude due to the reduction in surface charge. Coatedsystems showed a similar response to the reduction in surface charge.Coated systems showed a similar response to pH changes, indicatingelectrostatic stabilization as well. Increasing the temperature forslips with a minimum viscosity did not show a sudden viscosity increaseover the temperature range investigated (23° C.-95° C.). The latterwould occur for steric stabilized particles in an aqueous system,assuming enthalpic stabilization to be predominant. In addition,dispersions agglomerated and settled rapidly at or near the IEP of thecoated powder, again indicating predominantly electrostaticstabilization. The surfactants used were initially all monomericshort-chain-length complexes. These complexes are somewhat polymerizedupon dilution and pH adjustment in preparation of a working solution.Though no direct evidence was presented, a steric contribution to thestabilization of these systems can not be ruled out based on rheologydata.

The presence of the surfactant at the powder surface was shown by FTIR,ESCA, and TEM analysis. FTIR showed adsorption bands for coated powdersthat corresponded to a few strong bands (1600-1200 cm⁻¹) for the puresurfactant solutions, after subtraction of the solvent. TheOH-stretching band at 3743 cm⁻¹ was weakened and a new, weak bandappeared around 3010 cm⁻¹. This indicated interaction of surfactant(metal) ions with surface silanols. The weak band resulted probably fromperturbed surface hydroxyl groups.

Electron diffraction in the TEM indicated the presence of a crystallinephase, not present in as-received powders. Identification of thecrystalline phase was possible using ESCA binding energy data. Thecoated systems had the additive as a metal hydroxide on the surface. ForS1, for example, the O1s values were 532.6 eV with a shoulder at 531.1eV, indicating SiO₂ and Cr(OH)₃, respectively. The Cr2p values were577.3, from Cr(OH)₃, or 577.7 eV, indicating the interaction with theorganic group. The Si2p peak at 101.9 eV had a shoulder at 103.3 eV,from Si₃ N₄ and SiO₂, respectively. The oxidized and sintered systemshowed the shoulder for Si2p at 102.7 eV, probably due to siliconoxynitride. The O1s values were 532.5 and 531.6 eV and the Cr2p valuewas 577.1 eV. The latter two correspond to values for Cr₂ O₃. Theconversion to the metal oxides due to oxidation was confirmed by FTIRanalysis of oxidized, pure surfactants. Thermal analysis showed a weightloss and exothermic transition for the coated powders below 400 C.,indicating the combustion of the organic. No further weight lossoccurred after 24 hours at 500/675 C., indicating the completion of theoxidation treatment. The amount of residual carbon in the oxidized discswere less than 1 wt %.

The amount of additive introduced into the system was calculated for thevarious surfactants using the saturation concentrations obtained frompotentiometric titration and repeated centrifuging as well as usingESCA, quantitative SEM-microprobe analysis, and thermal analysis data.The calculated amounts were in close agreement and are presented inTable 1 as mg M₂ O₃ per gram of Si₃ N₄ powder. The number of surfacesites needed to accommodate the amounts of additive listed would be2.8-3.5 sites nm⁻².

                  TABLE 1                                                         ______________________________________                                        Data on surfactants (M-[organic]) investigated                                                                        mg M.sub.2 O.sub.3 /                  Surfactant                                                                             Label   M.    # M  organic                                                                              IEP  g Si.sub.3 N.sub.4                    ______________________________________                                        Methacrylic                                                                            S1      Cr    2    C.sub.4 H.sub.5 O.sub.2                                                              8.0  4.5 ± 0.5                          acid                                                                          Formate  S2      Cr    3    C.sub.6 H.sub.6 O.sub.13                                                             7.5  4.0 ± 0.8                          Lactate  S3      Al    3    C.sub.3 H.sub.5 O.sub.3                                                              8.1  4.0 ± 0.8                          ______________________________________                                    

Various systems were sintered at 1730°-1950° C. The results of theseexperiments are summarized in Tables 2 and 3. The green densities were52-58% of theoretical.

                                      TABLE 2                                     __________________________________________________________________________    Data on discs sintered for 8 min. at 1730 C. under 0.1 MPa Nitrogen                 2 wt %                                                                             2 wt %                                                                              4 wt %                                                                              4 wt %                                                                              4 wt %                                           System                                                                              MgO  MgO + S1                                                                            MgO   MgO + S1                                                                            MgO + S3                                         __________________________________________________________________________    Density                                                                             78.5 85.8  84.1  93.2  92.9                                             (% theor).                                                                    E-mod.                                                                              --   175 ± 15                                                                         --    240 ± 10                                                                         265 ± 9                                       (GNm.sup.-2)                                                                  __________________________________________________________________________

The rather small amount of additive that could be incorporated (Table 1)did not provide very high densification by pressureless sintering,making high pressure sintering the choice for very high densification.The coated systems did transform to the Beta phase, but the finalpressureless sintered density was limited to 62%th after 6 hours at 1750C. Systems S2 and S3, which were designated so as to introduce threemetal atoms per monomer, did not give better results. Themicrostructures showed rather coarse elongated Beta grains and mostlyopen porosity. However, substituting the surfactant for a fraction ofthe additives conventionally incorporated by ball-milling improvedsintering results.

Three systems, MgO, MgO-Al₂ O₃ and MgO-Cr₂ O₃, were selected andsintering cycles were taken from data in literature for these systems.The results in Table 2 show the increase in density with the addition ofS1 coated powder ball milled with MgO. Densification was limited bothbecause of the decomposition and evaporation of MgO and because thetotal amount of additive from S1 was limited. Increasing the amount ofMgO ball-milled into the coated system to 4 wt % resembles systemsreported in literature with maximum densities of 86%th. Again additionof S1 clearly gave better results. To see whether the increase in theamount of additive by 0.45 wt % from S1 caused this improvement, discswith 0.5 wt % Cr₂ O₃ and 4 wt % MgO were sintered as well. Densitieswere comparable with the 4 wt % MgO system. The use of S3 with 4 wt. %MgO also gave better results than only a 4 wt % gO ball-milled system.Comparing S1 and S3 showed that their effect on the MgO system wassimilar. In addition, the final density under the sintering conditionslisted was higher with 2 wt % MgO+S1 than with 4 wt % MgO. The phaseequilibrium diagrams did not predict an increased amount of liquidformation at 1730 C. for the S1 system.

The data in Table 3 shows that fully dense materials were obtained usingthe conditions and systems listed. The data listed in Table 3 iscomparable to or better than data reported in literature for HIPedsystems, containing considerably larger amounts of additives. SystemA+S3 sintered to only 95.6% th when the pH of the surfactant solutionwas not raised, causing a decrease in the amount of additive introduced.This indicated that for a 10 m² /g powder the amounts listed in Table 1approach the minimum amount of additive needed, if uniformlydistributed.

                  TABLE 3                                                         ______________________________________                                        Data on discs HIPed for 60 min. at 1950 C. under 200                          MPa. Nitrogen                                                                          Density  H.sub.v    E       K.sub.c                                  System   (%)      (GNm.sup.-2)                                                                             (GNm.sup.-2)                                                                          (MNm.sup.-3/2)                           ______________________________________                                        pwd A + S1                                                                             100      19.2 ± .6                                                                             315 ± 3                                                                            4.8                                      pwd B + S1                                                                             100      19.1 ± .5                                                                             305 ± 5                                                                            4.8                                      pwd A + S2                                                                             100      18.4 ± .4                                                                             300 ± 5                                                                            4.1                                      pwd A + S3                                                                             100      19.5 ± .5                                                                             297 ± 2                                                                            5.3                                      ______________________________________                                    

Referring to Table 4, there is shown comparative results ofdensification for green and sintered silicon nitride powders havingsintering aids incorporated therein mechanically, i.e. by ball milling,by the precipitation of the sintering aid in a dispersion of the ceramicpowder, and by coating in accordance with the present invention.

                  TABLE 4                                                         ______________________________________                                        Green and Sintered Densities for Compositions with                            ≦1 wt % Me.sub.x O.sub.y ; 6 hrs, 1750° C., 0-1 MPa             N.sub.2                                                                       Incorporated             Max. Green ρ                                                                        Sintered ρ                             by:      Me.sub.x O.sub.y                                                                      wt %    % Theoretical                                                                           % Theoretical                              ______________________________________                                        Ball Milling                                                                           MgO     1       54 ± 2  64 ± 1*                                         Al.sub.2 O.sub.3                                                                      1       54 ± 2 68 ± 1                                  Precipitation                                                                          Cr.sub.2 O.sub.3                                                                      1       51 ± 2 65 ± 1                                           MgO     1       50 ± 2 63 ± 1                                  Coating  Cr.sub.2 O.sub.3                                                                      0.45     59 ± 2**                                                                              73 ± 1***                                      Cr.sub.2 O.sub.3                                                                      1.1     59 ± 2 76 ± 1                                           Al.sub.2 O.sub.3                                                                      0.1     59 ± 2 65 ± 1                                  ______________________________________                                         *2 hrs, 1750° C.: 67 ± 1%                                           **CIP'ed: 61 ± 1%                                                          ***CIP'ed: 74 ± 1%                                                    

As can be seen from Table 4, the addition of small amounts of sinteringaid in accordance with the present invention resulted in higher densitygreen and sintered ceramic articles as compared with other methods ofincorporating even greater amounts of the sintering aids.

Table 5 tabulates the experimental results of certain properties ofvarious hot isostatically pressed silicon nitride powders treated inaccordance with the present invention and sintered at 1950° C. for 1hour at a pressure of 207 MPa. These powders were first densified to agreen state having a density of 59±2% of theoretical.

                                      TABLE 5                                     __________________________________________________________________________    Density (ρ), Vickers Hardness (H.sub.v), Elastic Modulus (E), and         Indentation                                                                   Toughness (K.sub.C) for HIP'ed Compositions; 1950° C., 1 hr, 207       MPa                                                                           (Green Densities 59 ± 2% Theoretical)                                               ρ H.sub.v                                                                             E    K.sub.C                                                                            MOR                                            Powder/Complex                                                                         % Theo.                                                                             GNm.sup.-2                                                                          GNm.sup.-2                                                                         MNm.sup.-3/2                                                                       MPa                                            __________________________________________________________________________    A/S1     100   18.6 ± 0.2                                                                       315  3.81 610 ± 5                                     B/S1     100   18.5 ± 0.2                                                                       305 ± 5                                                                         3.80 --                                             A/S2     99.3 ± 0.2                                                                       17.6 ± 0.3                                                                       293 ± 5                                                                         3.29 --                                             A/S3     100   19.1 ± 0.3                                                                       297 ± 2                                                                         4.02 --                                             A/Cr(NO.sub.3).sub.3                                                                   100   18.4 ± 0.3                                                                       308 ± 2                                                                         3.58 --                                             A        98.6  16.1 ± 0.4                                                                       286 ± 3                                                                         2.57 --                                             B/S3     100   19.0 ± 0.4                                                                       297 ± 3                                                                         5.00 591 ± 7                                     __________________________________________________________________________

What is claimed is:
 1. A treated silicon nitride ceramic powdercomprising powder particles having a substantially uniform monomoecularlayer of a sintering aid formed from the decomposition of anorganometallic sintering aid forming compound, chemisorbed on thesurface thereof, said sintering aid comprising a metal ion chemisorbedto the powder particle, said powder formed from a stable liquiddispersion of the powder having the sintering aid forming compounddissolved therein and wherein the sintering aid is selected fromchromium, magnesium and aluminum containing compounds and mixturesthereof.
 2. The ceramic powder recited in claim 1 wherein the metal ionis coupled to an anion selected from the group consisting of oxygen andnitrogen.
 3. The ceramic powder recited in claim 1 wherein theorganometallic is selected from the group consisting of a metal formate,a metal lactate, a metal acrylate and mixtures thereof.
 4. The ceramicpowder recited in claim 3, wherein said powder is a silicon nitride andsaid organometallic comprises at least one member of the groupconsisting of chromium methocrylate, chromium formate and aluminumlactate.